Gravitational Waves: Ripples in the Fabric of
Space Time

Albert Einstein predicted the existence of gravitational waves in 1916
as part of the theory of general relativity. Einstein described space
and time as different aspects of our reality, in which matter and energy
are ultimately the same thing. Space-time can be thought of as a "fabric"
defined by the measuring of distances by rulers and the measuring of time
by clocks. The presence of large amounts of mass or energy distort space-time--in
essence causing the fabric to become curved, or "warped"--and we observe
this as gravity. Freely falling objects-whether a soccer ball, a satellite,
or a beam of starlight-simply follow the most direct path in this curved
space-time.

When large masses move suddenly, some of this space-time curvature ripples
outward, the ripples spreading much the way ripples do on the surface
of a pond after a stone has been thrown into the water. Imagine two neutron
stars orbiting each other. A neutron star is the burned-out core that
can be left behind after a star explodes. It is an incredibly dense object
that can carry about as much mass as a star like our sun, in a ball only
a few miles wide. When two such dense objects orbit each other, space-time
is stirred by their motion, and gravitational energy ripples outward into
the universe.

In 1974 Joseph Taylor and Russell Hulse found such a pair of neutron
stars in our own galaxy. One of the neutron stars is a pulsar, meaning
that it beams regular pulses of radio waves toward Earth. Taylor and his
colleagues were able to use these radio pulses, like the ticks of a very
precise clock, to study the orbiting neutron stars. Over two decades,
they watched for and found the tell-tale shift in timing of these pulses,
which indicated the loss of energy from the stars-energy that had been
carried away as gravitational waves. The result was just what Einstein
had predicted it would be!

How LIGO Works

Diagram of LIGO Detector
(click for larger view)

LIGO will detect the ripples in space-time by using a device called a
laser interferometer, in which the time it takes light to travel between
suspended mirrors is measured with high precision using controlled laser
light. Two mirrors hang very far apart, forming an "arm" of the interferometer,
and two more mirrors make a second arm perpendicular to the first arm.
When viewed from above, the two arms form an L shape. Laser light enters
the arms through a beam splitter located at the corner of the L, dividing
the light between the arms. The light is allowed to bounce back and forth
between the mirrors many times before it returns to the beam splitter.
If the two arms have identical lengths, then interference between the
light beams returning to the beam splitter will direct all of the light
back toward the laser. But if there is any difference between the lengths
of the two arms, some light will travel to where it can be recorded by
a photodetector.

The space-time ripples cause the distance measured
by a light beam to change as the gravitational wave passes by, causing
the amount of light falling on the photodetector to vary. The photodetector
then produces a signal telling how the light falling on it changes over
time. The laser interferometer is like a microphone that converts gravitational
waves into electrical signals. Two interferometers of this kind are being
built for LIGO  one near Richland, Washington, and the other near
Baton Rouge, Louisiana. LIGO must have two widely separated detectors,
operated in unison, in order to rule out false signals and confirm that
a gravitational wave has passed through Earth.

Pushing The Limits of Technology

LIGO Mirror
(click for larger view)

LIGO must measure the movements of its mirrors (which are separated by
two and a half miles) with phenomenal precision. To achieve its goal,
LIGO must detect movements as small as one thousandth the diameter of
a proton, which is the nucleus of a hydrogen atom. Achieving this sensitivity
requires a remarkable combination of technological innovations in vacuum
technology, precision lasers, and advanced optical and mechanical systems.

LIGO's interferometers are the world's largest precision optical instruments.
As such, they are housed in one of the world's largest vacuum systems,
with a volume of nearly 300,000 cubic feet. The beam tubes and associated
chambers must be evacuated to a pressure of only one-trillionth of an
atmosphere, so that the laser beams can travel in a clear path, with a
minimum of scattering due to stray gases. To do this, LIGO scientists
and engineers have worked with industry to produce steel with a very low
dissolved hydrogen content.

Beam Splitter Vacuum Chamber
(click for larger view)

The LIGO laser light comes from high-power, solid-state lasers that must
be so well regulated, that over one hundredth of a second, the frequency
will vary by less than a few millionths of a cycle. This severe requirement
places the LIGO detectors among the most precise test beds available for
laser stabilization and attracts significant laser development activity
worldwide.

The suspended mirrors, must be so well shielded from vibration that the
fundamental motion of the atoms within the mirrors and suspension fibers
can be detected. The high precision vibration-isolation systems needed
to achieve this are very closely related to equipment used for the masking
and etching of circuitry on silicon in semiconductor manufacturing.

Vibration Isolation Coiled Springs
(click for larger view)

More than thirty different control systems are required
to hold all of the lasers and mirrors in proper alignment and position,
to within a tiny fraction of a wavelength, over the four kilometer lengths
of both arms of the interferometers . These control systems must be monitored
continuously and able to function without human intervention. Sophisticated
simulation software and state-of-the-art electronics design are being
developed to perform these tasks.

LIGO: A New Way to Explore the Universe

Imagine watching a concert on television with the volume turned down.
The rousing score can only be imagined. Could we, in fact, even imagine
the music if we had never heard music before? Throughout human history,
we have viewed the heavens in a similar way. First with our unaided eyes,
then with telescopes, we viewed the visible light from heavenly objects
to learn their secrets. Eventually we learned to view a broader variety
of radiation, such as infrared light, x-rays, gamma rays, and radio waves,
which are invisible to our eyes,but are detectable by electronic devices.
But all of these different kinds of radiation, including light, are made
up purely of electricity and magnetism.

Today we know that only about 10 percent of all the matter in the universe
can be observed in this way. How else might we gain insight into the majority
of matter in the universe? We now have the technology to use a very different
force, the force of gravity, to explore the heavens. LIGO (for Laser Interferometer
Gravitational-Wave Observatory) is an instrument for sensing the presence
of matter, whether shining or dark, in the distant reaches of the cosmos.
LIGO will do this by detecting the gravitational waves  ripples
in the fabric of space-time  created by violent events such as the
collisions of stars and the vibrations of black holes.

Imagine now turning up the volume on that televised concert performance
and hearing the stirring sounds of a symphony. What a difference it makes
to experience music with this new sense! What new experiences await us
when we begin exploring the heavens with LIGO?

LIGO: An Observatory for the 21st Century

LIGO should start the new millennium by directly detecting gravitational
waves for the first time, perhaps recording the final death spiral of
two orbiting neutron stars just before they collide and merge into one.
Physicists have predicted that such an event will produce a burst of gravitational
waves with a characteristic pattern, its own fingerprint, that LIGO should
be able to detect and measure, initially out to distances of 70 million
light-years. As has happened so often, when we enter a new domain of measurement,
totally unexpected discoveries could surprise us. Improved detectors will
look deeper into the universe and detect more exotic events.

Science like this is the epitome of basic research. As always with basic
work, no one knows where it will lead or what its consequences and ramifications
will be. For example, 19th-century scientists classified the spectral
lines found in sunlight because it was interesting, having no idea that
a century later their work would lead to the understanding of atomic structure
and the development of quantum mechanics. In turn, the inventors of the
laser built upon the foundation of quantum mechanics, never imagining
that their invention would be used for delicate eye-saving surgery, in
supermarket checkout counters, for printing daily newspapers, or as a
light source for LIGO.

Will the discoveries made by LIGO have such an impact? The hope is that
they will, but the experiments must be done first!

There is major involvement with other universities and institutions besides
Caltech and MIT, both within the United States and abroad. The LIGO
Scientific Collaboration (LSC) has been organized to foster such participation.
It offers a mechanism for two-way communication about design decisions
today and about science program decisions in the future.

LIGO will strongly support science education and other educational activities
in the states and communities where the observatories are located. The
resident staff at the Washington State and Louisiana observatories, as
well as the steady stream of top scientists visiting from all over the
globe, will contribute to the intellectual and cultural life of the local
communities.